Offset Image Brightness
I don't think an automatic way currely exists. I'm afraid you have to code something to find the target pixel directly.

However, after playing this for quite a bit time, I find a tricky way to get the same result. As shown in the image below. Though it seems not quite effective on performance, but quite acceptable after my testing on a sort of 6000 x 6000 px image, so far.



Furthermore, suppose you want to target the brightest pixel, just add an Invert node right before the Nomalize node. Hope that helps.



Version 2:

I think this way is even better. FYI



OR: